My expansion tank recently spouted numerous leaks. It was supposedly new (changed about 2 months ago) but I can see that the cap wasn't changed.

I live at 8500ft above sea level - the boiling point of water up here is about 73C and the M62 thermostat doesn't even open until about 105C - could this cause a problem with pressure in the system thereby making the expansion tank leak?

I've put in an 88C thermostat but am still waiting on the new tank/cap to see if there's any change.

The coolant system is a closed pressurised system with the pressure determined and controlled by the radiator cap.
Whether you are at 1000ft or 10000ft should have no affect on the pressure in the system.
The engine thermostat temperature has no bearing on the system pressure.
Whatever the reason for the tank leaks it's not the boiling point of coolant or system pressure.

So the idiot mechanic who used the same cap is to blame? (assuming he even changed the tank)

I can't really say that. It's not inconceivable, but unlikely, the cap has failed in the closed position thus allowing the system to over-pressurise or it could even be the wrong cap in terms of pressure rating.
I would have expected a new cap to be fitted when the tank was replaced.

Your expectations of Kenyan mechanics are slightly higher than mine, even though I used a ridiculously expensive German owned garage.

I know the same cap was used - it has some damage to it and the damage was still there after I got it back.

Considering I also got the car back with the air filter tied on with wires (which were digging into the oil pipes), air intake hose completely disconnected from the throttle body and the cooling system filled with water instead of coolant I think it's fair to question whether the leaky old tank was even changed. sigh.

Seems your garage provides a service fairly comparable with that provided by a good few Main Dealerships in the UK.

Having had plenty experience with West African Sub Saharan mechanics (Nigeria) I share your scepticism on the quality of your mechanic. Having said that, plenty of idiots in the Netherlands (my home country) as well tbh.

Anyway, as for the cap blowing. I assume you can clearly see it came from the cap and the coolant is clearly coolant with no additional crap like sludge or oil ????? I had both heat exchangers (engine oil and ATF) fail on my 2006 4.2 SC and that caused pressure increase AND mixing of oil / ATF and coolant.

If your fluids are clean (ATF mixed, engine oil didn't) then it is possible the cap is to blame.

Altitude does actually make a difference as the caps work on differential pressure. So if the ambient pressure on the cap is less, it will open sooner as most caps are spring loaded AND directly connected to the environment. However, they should be designed for altitude as well so it should not do this unless the cap is not working properly or there is excess pressure building up in the system.

And the coolers on my 4.2 failed because our mechanics (the company I worked for has their own mechanics maintaining the fleet of cars) filled my cooling system with WATER. So needless to say that once I found out, I had them drain the system AGAIN to now fill it with coolant. IDIOTS. MY 2010 5.0 SC Galway green and sand interior!!
Have the Faultmate MSV2 Extreme to be tinkering with the settings etc. !!

The coolant is just coolant. I think. (i haven't noticed any oil in it though I did get a lot of sediment out when I drained it. Old pipes coated with scale.

Now that you mention ATF - the same mechanic also replaced the plastic connect between radiator and transmission oil cooler.

I haven't noticed it coming from the cap, rather it looks like it's weeping out of the walls of the expansion tank. The crack itself is invisible and the only thing that marks its location when the car is cold is the dry coolant. I've been driving like this and topping up for a few weeks but recently we got a new leak which is more like a stream of pee and it's losing coolant too fast to be driveable.

I assume the leak is in the expansion tank ?? If so they are easy to replace (well on a 2006 SC there are, I replaced it myself when that one started to leak). But if your mechanic hasn't even done that (standard Nigerian practice, that is why I used to supply the parts and let them show the parts coming of as well and I dispose of them myself) than he needs a big kick up the behind. A bit of shouting usually makes them do their work properly (for a while).

To be fair, I loved working with Nigerians and most of them did a good job, however there are also some that never learn and always try to scam their way through life, thinking they will get rich quick (and never do). But that is beside the point.

A standard trick in Nigeria was providing second hand parts from another car and selling your parts to someone else, unless you are really on top of them and then they just do the work. Heard many stories from people that had their car come back with parts replaced with worst parts (not even the part that was supposed to be replaced), hence our company having their own mechanics and buying their own parts (although the tyre scandal showed that there were still ways to cheat the boss out of money).

So my suggestion is, to build up a good relationship with the UK dealer/parts supplier and get the stuff shipped to you. That way you have OEM parts and then you still need to find a mechanic that can do a proper job . MY 2010 5.0 SC Galway green and sand interior!!
Have the Faultmate MSV2 Extreme to be tinkering with the settings etc. !!

I've just started doing all the work myself. I know a few honest mechanics but I don't necessarily trust them with a modern Range Rover.

Problem is even the honest (i.e. will give full disclosure about the parts' quality) have a habit of doing what we call 'jua kali' work. They lose bolts and use zip ties, tie things on with bits of wire (usually their inept assistants). When I did the oil change I found that the sump bolt had 2 copper washers on it and was wrapped in plumbers tape to stop it leaking.

The only really good mechanics around are either specialists - lot of rally drivers have garages which only deal with Subaru/Mitsubishi, or they're in awkward locations and would involve 3 hours of sitting in traffic to get to.

This guy was a (very stuck up) german, not a local. Only used him because he's close by, traffic is terrible and he said he'd do it in 3 days. He also has a decent reputation, although that doesn't count for much considering most people don't know what goes on in their car. I've found that a lot of 'good' reputations come from being overpriced and arrogant. If I was the average housewife I'd have been back there in a few months with an oil leak and/or a dead engine and he'd tut tut, comfort me and charge me for an engine rebuild that he caused.

Charging me full price but putting in a used part from a breaker is what I expect he's done. When he told me the IIDtool was a waste of time but then tried to sell me the EAStool for $1000 I should have known lol.

Kenya is relatively honest in that the quality you get generally depends on what you want to pay. When I was in Tanzania anything that wasn't nailed down would disappear. We had a planetary gear from a Scania truck in a locked warehouse in a locked compound with electric fence & gate wander off by itself.